1. Field of the Invention
The invention relates to hydraulic cylinder devices. In particular, the invention concerns long stroke, large bore, low friction hydraulic actuators. (Herein, "Friction" refers to "turn-around friction".)
Hydraulic actuators having cylinder bores approaching two inches, and piston strokes of twelve inches are fairly standard devices. (Piston stroke may be defined as the maximum excursion of the piston rod from the cylinder). In motion system simulators using hydraulic actuators, it is not uncommon to find hydraulic cylinders having bores in excess of two inches and stroke lengths as great as sixty inches or more. In general it may be said that cylinders having bores in excess of two inches and stroke lengths of eighteen inches or greater may be defined as large bore, long stroke hydraulic actuators.
2. Prior Art
In prior art actuators, friction levels encountered in operation of good devices have typically been on the order of three hundred pounds and in many instances as high as six to twelve hundred pounds.
With long stroke actuators, and especially where rapid extension and retraction of the piston is required, it becomes necessary to provide some form of safety device to prevent catastrophic failures of the system resulting from an uncontrolled deceleration after such extension or retraction.
Various techniques have been used to control the deceleration of the cylinder piston. External controls are often used to control the hydraulic fluid flow. Check valves and flow control valves may be employed in meter type circuitry. An advantage of the external controls is that deceleration can be regulated over a longer portion of the cylinder stroke. There is a disadvantage however in that the entire system is more expensive, because more components are required, and also because it is less reliable for having those additional components. It has also been found in practice that flow control valves often cause very unpredictable and unstable performance; whereas, check valves in the system may cause extensive pressure pulses when they change state.
Deceleration may be controlled with internal cushioning. Cushioning can be provided, and usually is, at both ends of the cylinder. Cushioning at one end slows the piston as it approaches full retraction and at the other end it slows the piston as it nears full extension. The typical internal cushion comprises a spear and cavity arrangement. When the piston moves, hydraulic fluid is forced out in front of its motion and exits through the cavity. As the piston approaches the end of its travel, the spear enters and begins to block part of the cavity opening. The blocking effect continues as the spear penetrates further into the cushion cavity. As the blockage increases, less fluid flows from in front of the piston into and through the cavity. This exerts a cushioning force on the piston, gradually slowing it, and stopping it when the spear completely fills the cavity.
While such spear and cavity cushions can be quite effective when initially put into operation, wear and scoring is caused by interference between the walls of the cavity and the spear. Thus, effective deceleration degrades over the useful lifetime of the actuator as the gradual wearing decreases the effective blockage, especially when using low viscosity fluid.
In many instances it is desired to know the displacement of the piston from some "home" position. To this end, various external metering devices have been attached to the cylinder and the piston rod to measure the displacement of the piston. These devices are often subject to damage, leading to inaccuracies, as they are subject to wear and to damage in their operational environment.
The invention has as its objective the production of a large bore, long stroke, low friction hydraulic actuator having friction levels of one hundred pounds or less and utilizing internal deceleration cushioning which is not subject to wear and degradation during operation.
It is a further object of the invention to provide an internal metering device for determining the displacement of the piston from said known reference position. Further objectives and advantages of the invention will be readily determined from the disclosure which follows.